The numerical aperture of an optical system is briefly described hereinafter to facilitate understanding of the conventional technologies concerned.
In an optical system designed to have little aberration according to geometrical optics, a focused spot must in theory be infinitely small in size. However, it has, in fact, a spatial spread in a finite size due to the effect of optical diffraction owing to the wave motion characteristic of light.
Now, provided that a numerical aperture of an optical system, contributing to optical image formation or condensing of light, is designated NA, the spatial spread of a focused spot is defined by the following formula: EQU k.times..lambda..div.NA (1)
where .lambda.=light wavelength
k=a constant for respective optical systems (a value, normally, in the range from 1 to around 2). Further, the numerical aperture NA is proportional to a ratio of the diameter D of an effective entrance pupil of an optical system (generally the diameter of an effective light beam) to a focal length f, that is: D/f.
The spatial spread of the focused spot as expressed by the formula given above represents a theoretical resolution limit of the optical system, and is also called a diffraction limit.
As is evident from the above formula, a theoretical resolution may be enhanced by use of a light beam at a shorter wavelength .lambda., or by enlarging the numerical aperture NA of an optical system. However, a short wavelength light source is generally complex in construction, and higher in production cost.
Particularly, in the case of a laser light source used for optical disc systems, photolithographic masking systems, and the like, this tendency becomes more pronounced. Further, the greater the numerical aperture of an optical system, the more the optical system becomes prone to have aberration due to geometrical optics. Accordingly, for recording information on common optical disc systems, a semiconductor laser for emitting a light beam at a wavelength on the order of 700 nm is used as a light source while a condensing optics having the numerical aperture NA on the order of 0.5 is used.
As the conventional technology capable of achieving superresolution by use of the light source and condensing optics described above, a superresolving optical system constructed such that a portion of an effective light beam falling on the condensing optics is shielded with a shading band is well known (reference: Japanese Journal of Applied Physics, Vol. No. 28 (1989) Supplement 28-3, pp. 197-200). It appears that with this superresolving optical system using the shading band, a focused spot size is rendered narrower by 10 to 20% with respect to the theoretical resolution limit of the optical system.
However, shielding a portion of the effective light beam falling on the condensing optics by means of the shading band will result in a lower optical utilization rate. Furthermore, with the superresolving optical system described above wherein the central region of a light beam, including the optical axis, is shielded with the shading band, degradation in the optical utilization ratio becomes further pronounced because the central region of the light beam generally belongs to a high intensity zone according to the distribution of light intensity.
Such a low optical utilization rate inevitably requires use of a light source capable of outputting higher power, resulting in a higher cost of an optical apparatus because such a high power output light source is expensive. Particularly, for application to optical disc systems, a semiconductor laser light source, expensive even at low power output, is used, and consequently, it is practically impossible to employ a high power output light source from the cost point of view.
The invention has been developed in light of the circumstances described above, and a main object thereof is to realize superresolution without sacrificing optical utilization ratio.